While a traditional concern of any logging operation is the efficient transportation of felled timber from a forest to processing plants, modern logging planners are also concerned with minimizing safety hazards and environmental damage resulting from such operations. Thus, while clear-cutting timber may be the most "efficient" way to log a forest, logging planners may opt to selectively harvest timber because of environmental or timber management concerns.
It is important, therefore, that modern logging equipment be designed with the broadened concerns of logging planners in mind. Ideally, logging equipment will be adaptable for use in either clear-cutting or selective harvesting situations. Additionally, it is desirable to use logging equipment which will minimize the disruption of the soil in the area being logged. Such disruption can often result in excessive soil erosion, which will not only be detrimental to the forest land but can adversely impact aquatic life forms in nearby streams.
After timber is harvested, the resulting logs are transported to a landing. A landing is a generally level area, situated near a logging road, at which logs are loaded on trucks for hauling to processing plants. The act or process of conveying logs to a landing is known as "yarding." When timber is harvested on grades of less than 25-35%, tractors or other heavy equipment, such as skidders, may be used for yarding. Such equipment is generally efficient only at haul distances below 1,000 feet and works best in clear-cutting operations. When harvesting steeper slopes or hauling over longer distances, some type of cable yarding system is often employed.
One such system is a prior art skyline system 10 shown in FIG. 1. As is typical, the skyline system 10 includes a first portable tower 12 from which a skyline cable 14 extends to a second, typically shorter, portable tower 16. A skyline carriage 18 with grooved wheels 20 rides on the skyline cable 14 to carry logs to a landing positioned near the second portable tower. A second cable, known as a mainline cable 22, extends from the first portable tower 12 to the skyline carriage 18. Also attached to the skyline carriage 18 is a grapple cable 24 that extends downward from the motor carriage for attachment to the logs such as by grapple hooks 25.
The first portable tower 12 includes a first winch 26 to which the skyline cable 14 is connected. The first winch 26 pays out the skyline cable 14 to lower the skyline carriage 18 and reels in the skyline cable to raise the motor carriage. The first portable tower 12 also includes a second winch 28 to which the mainline cable 22 is connected. The second winch 28 reels in the mainline cable 22 to pull the skyline carriage 18 toward the first portable tower 12 and pays out the mainline cable to allow the motor carriage to travel toward the second portable tower 16 by gravity.
An important characteristic of a skyline system, such as the skyline system 10 shown in FIG. 1, is its lateral reach or yarding width. In the skyline system 10, logs positioned laterally away from the skyline cable are accessed by a logger pulling the grapple cable 24 laterally to the logs with the skyline cable 14 payed out to slacken the cable as necessary to reach the logs. After the logger attaches the grapple cable 24 to the logs, the skyline cable 14 is reeled in to cause the logs to swing to a position underneath the skyline carriage 18. The logs are then carried to the landing by the skyline carriage. Such a system of manually pulling the grapple cable laterally of the skyline cable 14, attaching the grapple cable to the logs, and allowing the logs to swing underneath the skyline cable poses many dangers to the loggers. In addition, such a procedure is time consuming, and thus, limits the speed of the logging operation.